In recent years, CMOS image sensors are widely used for the purposes such as a digital still camera, a camcorder, and a surveillance camera, and the market of CMOS image sensors is expanding.
A CMOS image sensor converts light incident upon each pixel into electron using a photodiode which is a photoelectric conversion device, and accumulates the electron for a certain period of time, and the CMOS image sensor converts a signal reflecting the amount of accumulated electrical charge into digital and outputs the signal.
In general, a pixel circuit of the CMOS image sensor uses a source follower incorporated into the pixel circuit to convert an electrical charge signal given by the photodiode into a potential signal of an output (vertical) signal line and output the signal.
During reading, selection of pixels is executed in order in units of rows, and the pixel signals of columns in the selected row are converted from analog into digital (A/D) in order or in parallel, and the signals are output as captured image data.
In particular, in recent years, an A/D conversion device is provided for each column in order to increase the speed and the conversion is performed at a time in many cases.
The A/D conversion of the CMOS image sensor widely uses a slope A/D conversion device in which a comparator compares an image signal with a ramp wave serving as a reference signal (reference voltage), and a counter performs time integration until the output of the comparator is inverted.
The slope A/D conversion device has superior linearity and noise characteristics.
A column A/D conversion device including multiple A/D conversion devices arranged for corresponding pixel columns and performing A/D conversion at a time can improve the speed because the operating frequency per A/D conversion device is reduced. In addition, the reference voltage generation device is shared by the A/D conversion devices, and therefore, the footprint and the power consumption efficiency are advantageous, and the compatibility with CMOS image sensors is better as compared with other A/D conversion methods.
Patent Document 1 discloses the following technique.
In this technique, multiple reference voltage generation devices are provided to generate different ramp wave inclinations for a single comparator, and captured images are obtained such that data are obtained with a high bit precision in a dark place, and data are obtained with a low bit precision in a bright place, whereby an image with a high dynamic range is obtained.
This technique makes use of the feature of image sensing that a fine level of gradation is required only in a dark place.
On the other hand, Patent Documents 2, 3 disclose the following technique.
In this technique, in a column A/D conversion device, gray code counters and latches provided for multiple corresponding columns obtain lower bits, and binary ripple counters provided for multiple corresponding columns obtain higher bits, so that the power consumption is greatly reduced.
In this technique, in the higher bit, digital CDS (Correlated Double Sampling) is achieved by up/down counting performed by a ripple counter according to conventional techniques.
Since the lower bit is a gray code, black level and image data are stored to the latches, and the data are transferred to a digital processor and the like, in which the data are converted into binary codes, and are subjected to digital CDS.